Coherently translate from Spanish to English: Proteins, whose term comes from the Greek "proteios," meaning fundamental or essential, are indispensable elements for the proper development and functioning of the cells that make up the organism (Chuan, 2021). These biomolecules, primarily composed of carbon, hydrogen, oxygen, and nitrogen, may also contain other chemical elements such as sulfur, phosphorus, iron, magnesium, or copper in certain protein structures (Sánchez Amador, 2020). Proteins exhibit great structural and functional diversity and can be classified into different types based on their role in the organism (Chuan, 2021). In line with Chuan (2021), some proteins act as hormones, such as insulin, which regulates blood sugar levels; others function as enzymes, for example, lipases, involved in digestive processes; and others play a defensive role, like antibodies, which combat infections.
From a physiological standpoint, proteins are the main components of the cell and are necessary for tissue repair, growth, cell division, and many other functions related to the physical structure of living beings (Sánchez Amador, 2020). Therefore, it is recommended that 10% to 15% of any human diet be composed of proteins (Sánchez Amador, 2020). These essential macromolecules for life are found in various foods, such as salmon, eggs, milk, legumes, beef, and many more (Sánchez Amador, 2020). According to Koshland & Haurowitz (2024), chemists recognized the importance of proteins in the early 19th century, and the Swedish chemist Jöns Jacob Berzelius coined the term "protein" in 1838.
Amino Acids: The Essence of Proteins
Proteins are composed of basic units called amino acids (Sánchez Amador, 2020). These molecules have a chemical structure consisting of a central carbon, an amino group, a carboxyl group, a hydrogen atom, and a variable chemical side chain. In this way, they resemble a chemical "cross" with a tetrahedral structure in three-dimensional space. According to Sánchez Amador (2020), there are twenty different amino acids that combine in various ways through peptide bonds to form proteins (polypeptide chains) found in nature.
Amino acids can be classified into two types: essential and non-essential (Sánchez Amador, 2020). Essential amino acids are those that the body cannot synthesize and must therefore be obtained through the diet (Sánchez Amador, 2020). It is important to note that the "essentiality" of each amino acid depends on the type of living organism considered, as different metabolic pathways in species make some require certain compounds that others do not. On the other hand, non-essential amino acids are those the body can produce (mainly in the liver) from intermediates through transamination and are therefore not essential in the diet. Finally, according to Sánchez Amador (2020), there are conditionally essential amino acids, meaning they are needed in daily intake in specific contexts and situations.
Structure
Formed by linear chains of amino acids, proteins are large macromolecules that can have between a hundred and three hundred of these components, or even more (Chuan, 2021). The specific function of each protein in the body depends on the amino acids that constitute it and their order (Chuan, 2021). Although not all the functions of a protein can be explained based on its amino acid sequence, Koshland & Haurowitz (2024) mention that the properties of these biomolecules determine the correlations between the structure and function of proteins.
Proteins share a common central chemical framework, consisting of a linear chain of amino acids (Sánchez Amador, 2020). This is called the "primary structure," and it conditions the nature and function of the protein in the body (Sánchez Amador, 2020). According to Koshland & Haurowitz (2024), the primary structure of a protein is defined by its sequence of amino acids, without considering the spatial arrangement of the peptide chain.
However, proteins also have other more complex structures resulting from the folding of the macromolecule, its three-dimensional configuration, and other factors (Sánchez Amador, 2020). Thus, secondary, tertiary, and quaternary structures are distinguished (Sánchez Amador, 2020). The secondary structure refers to the spatial arrangement of the main peptide chain, without considering the side chains or other segments (Koshland & Haurowitz, 2024). The tertiary structure relates to the conformation of the side chains and other adjacent segments of the main chain, without considering neighboring peptide chains. Finally, according to Koshland & Haurowitz (2024), the quaternary structure applies to the arrangement of identical or different subunits of a large protein, where each subunit is a separate peptide chain.
Functions
Proteins vary according to the species and organ of origin (Koshland & Haurowitz, 2024). For example, the proteins found in an organism's muscles are not the same as those found in its brain or liver (Koshland & Haurowitz, 2024). However, according to Sánchez Amador (2020), all proteins have essential functions for the development and maintenance of cells and can be classified based on the tasks they perform.
Among the most important functions of proteins are catalysis, regulation, protection, and structure (Sánchez Amador, 2020). Catalysis refers to the ability of proteins, especially enzymes, to accelerate the chemical reactions that occur in the body. Regulation is related to the role of proteins, such as hormones, in maintaining the balance of the organism and intervening in various physical and behavioral functions. Protection is due to the immunological function of proteins, such as antibodies, which defend the body against external agents. Structure is based on the formation of proteins, such as collagen, tubulin, and keratin, which constitute the physical parts that characterize living beings. In addition to these functions, proteins can also act as carriers, motors, pigments, energy sources, and more. In conclusion, according to Sánchez Amador (2020), proteins are indispensable for almost all biological processes in life.
Classification
According to their Origin
Proteins can be classified based on their origin into two main groups: those of animal origin and those of plant origin (Corbin, 2016). Animal proteins are obtained from animals, such as eggs or pork. According to Corbin (2016), plant proteins are those obtained from plants, such as legumes, wheat flour, or nuts.
According to their Function
Proteins can be classified based on their function into different types (Corbin, 2016). On one hand, there are hormonal proteins, secreted by endocrine glands, acting as chemical messengers between cells, regulating various physiological processes. On the other hand, there are enzymatic or catalytic proteins that accelerate metabolic reactions in cells, facilitating functions such as digestion, liver detoxification, or the conversion of glycogen into glucose. Similarly, there are structural proteins, also known as fibrous proteins, which are part of tissues and organs, providing them with strength and elasticity. According to Corbin (2016), examples include collagen, keratin, and elastin, found in connective, bone, cartilaginous, capillary, nail, dental, and skin tissues.
Other important proteins include defensive proteins, with immune or antibody functions, protecting the body from infections by bacteria, viruses, and other pathogenic microorganisms (Corbin, 2016). These proteins are produced in white blood cells and are responsible for recognizing and neutralizing invaders. Additionally, there are storage proteins, which store essential mineral ions such as potassium or iron for cellular balance. There are also transport proteins, dedicated to carrying minerals and other substances to cells, such as hemoglobin, transporting oxygen from the lungs to tissues. Furthermore, there are receptor proteins located in the cell membrane, controlling the passage of substances into the cell, such as GABAergic neurons, which have specific protein receptors for the neurotransmitter GABA. Finally, according to Corbin (2016), there are contractile or motor proteins that regulate the movement and force of the heart and muscles.
According to their Conformation
Proteins are molecules with a specific three-dimensional orientation in space, determined by the rotation of characteristic groups (Corbin, 2016). According to this conformation, proteins can be classified into two main types: fibrous proteins and globular proteins. Fibrous proteins are characterized by having polypeptide chains aligned in parallel, forming resistant and water-insoluble structures. Examples of these proteins include collagen and keratin. In contrast, globular proteins are characterized by having polypeptide chains coiled upon themselves, forming spherical and water-soluble structures. According to Corbin (2016), these proteins often have transport functions, such as hemoglobin, which carries oxygen from the lungs to tissues.
According to their Composition
Finally, proteins can also be classified based on their composition, i.e., the components that make them up (Corbin, 2016). According to this criterion, proteins can be divided into two groups: holoproteins or simple proteins, and heteroproteins or conjugated proteins. Holoproteins are formed solely by amino acids. Heteroproteins are those that, in addition to amino acids, contain other non-amino acid components called prosthetic groups. According to Corbin (2016), examples of heteroproteins include glycoproteins, which have sugars in their structure; lipoproteins, which contain lipids; nucleoproteins, attached to nucleic acids like chromosomes and ribosomes; metalloproteins, containing one or more metal ions, like some enzymes; and hemoproteins or chromoproteins, having a heme group, such as hemoglobin.
References
Chuan, A. (2021, agosto 5). Proteínas de Alto Valor Biológico: Qué son, Características y sus Fuentes. Psicología y Mente. https://psicologiaymente.com/nutricion/proteinas-alto-valor-biologico
Corbin, J. A. (2016, noviembre 1). Los 20 Tipos de Proteínas y sus Funciones en el Organismo. Psicología y Mente. https://psicologiaymente.com/nutricion/tipos-de-proteinas
Koshland, D. E., & Haurowitz, F. (2024). Protein. En Encyclopedia Britannica. https://www.britannica.com/science/protein
Sánchez Amador, S. A. (2020, octubre 14). Proteínas: Qué son y Cómo Influyen en el Funcionamiento del Organismo. Psicología y Mente. https://psicologiaymente.com/nutricion/proteinas
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